Electric cable and method of making

ABSTRACT

In an electric cable having a semiconducting jacket applied directly over a wall of polymeric insulation, excessive bonding of the jacket to the insulation is prevented by applying the curing agent for the jacket only to its outer surface after extrusion, and then effecting a vulcanization such that the inner jacket surface remains thermoplastic.

United States Patentv Wade v 1451 Dec. 5, 1972 1541 ELECTRIC CABLE ANDMETHOD OF, I

MAKING 1 v [72] Inventor: 'RobertM. Wade, Wabash, Ind.

[73] Assignee: Anaconda Wire and Cable Company, New York, NY.

[22] Filed: March 6, 1972 [21] Appl. No.: 231,840

[52] US. Cl. ..174/115, l17/75, l74/l 10 PM,

, '174/12osc [51] -lnt.Cl. ..H01b7 /02,11o1b9/02- [58] Field of Search.174/120 R, 1 20 C,v 1-20 SC, 120 SR,l74/l 15, 110 PM; 117/69, 72,7 5;252/637,

[56] References Cited UNITED STATES PATENTS 3,666,877 5/1972Arnaudimlr.etal....,. ;.,.i7 i/l2OSC Ling et al ..174/12o sc 3,646,2482/1972 3,614,300 10/1971 Wilson .......l74/l20R 3,571,613 3/1971 Plate..174/11sx 3,474,189 10/1969 Plate et al ..174/11s 3,541,228 11/1970Lombardi ..174/12o SC Arnaudin, Jr. et al. 174/120 R PrimaryExaminer-Lewis H. Myers Assistant Examiner-A. T. Grimley AttorneyVictorF. Volk [57] ABSTRACT In an electric cable having a semiconductingjacket applied directly over a wall of polymeric insulation, excessivebonding of the jacket to the insulation is prevented by applying thecuring agent for the jacket only toits outer surface after extrusion,and then effecting a vulcanization such that the inner jacket surfaceremains thermoplastic.

, v 24 Claims, 2 Drawing Figures 1 j ELECTRIC CABLEAND METHOD OF MAKINGBACKGROUND OF THE INVENTION Economies in high-voltage cables haverecently been effected by eliminating the separate'shielding system,making the protective jacket semiconducting,'and applying it directlyonto the wall of cable dielectric. Such a cable construction, whereinthe jacket comprises embedded drain wires is described'in Plate et al.U.S. Pat. No. 3,474,189, the disclosures of which are included herein byreference. Although it would be-desirable to increase the toughness andtensile strength of the jacket material by vulcanization, this has notproven feasible because of the necessity .of avoiding adhesion betweenthe semiconducting jacket and the outer surface of the wall ofinsulation. Such adhesion would make it difficult to free stripped endsof cablefrom residues of the electrically conducting jacket compositionat terminations and joints.

SUMMARY BRIEF DESCRIPTION OF THE DRAWING FIG. 1 shows a section of acable made to my invention FIG. 2 shows .the steps ofa method of myinvention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Referring to FIG. 1, acable of my invention, indicated generally by the numeral 10, has aconductor 11, a heavy wall 12 of insulation, and a semiconducting jacket13 in which are embedded a plurality of drain wires 14. The wall 12comprises cross-linked polyethylene, but my invention has application tocables with insulation walls of other compositionssuch,

for example, as ethylene-propylene rubber, polyvinyl chloride, andunvulcanized polyolefin. However, my invention has particular advantagesfor cross-linked polyethylene insulated cables since cross-linkedpolyethylene has a particularly high thermal coeff cient of expansion,and this places an extra strain on the jacket 13. The jacket 13comprises chlorinated polyethylene blended with ethylene ethyl acrylateand a high proportion of carbon black to increase its electricalconductivity. A preferred thermoplastic jacket has been described inapplication Ser. No. 167,741 assigned to the assignees of the presentinvention, which has the advantage, that, being thermoplastic, it doesnot bond unduly to the wall 12. In the present invention an innersurface 16 of the jacket 13 remains thermoplastic but an outside surface17 has been vulcanized to provide greater toughness and tensilestrength. vulcanization of the outer portion of the jacket 13 isachieved by diffusion of vulcanizing agent through the surface 17 sothat the jacket 13. is homogeneous except for radial differences in thedegree of vulcanization or cross linking. This vulcanization decreasesgradually through a radial-section of the jacket 13 so as to leave theinner surface 16 free from vulcanization. Chlorinated polyethylene hasbeen emin two ways. The higher tensile strength of the vulcanizedcomposition resists radial expulsion of the wires by electrical forcesduring surges of current, and the higher softening temperature of thevulcanized composition prevents the surface from melting during shortperiods of high ambient temperature.

In FIG. 2 I have diagrammed a method of making the cable 10 whereby areel 21 of the conductor 11 is continuously paid through an extruder 22,a vulcanizing tube 23, and cooling section 24 for application of thedielectric wall 12. The insulated core then passes through a secondextruder 26 wherein it receives the jacket 13 and drain wires 14.

So far the description of my method has been previously disclosed forthe manufacture of cables with thermoplastic jackets. I now provide anovel step of coating the jacket 13 with a suitable curing agent bypassing the cable 10 through a chamber 28 wherein it is sprayed orotherwise coated with di-a-cumyl peroxide. Although I prefer to usedi-a-cumyl other peroxides are known for curing compositions comprisingchlorinated polyethylene and polyethylene and these may be used withinthe scope of my invention, including tertiary peroxides in which thevalences of the carbon atoms not linked to oxygen are attached to alkyl,cycloalkyl, alkylcycloalkyl, cycloalkyl-alkyl, aryl or aralkyl radicals,and also quinhydrone dimerides as disclosed in U.S. Pat. No. 3,036,982.Solvents other than acetone may be used, less volatile solvents havingthe advantage of holding the vulcanizing agent on the surface 17 throughthe entrance to a curing chamber. It is not essential to my invention,however, to apply the curing agent in solution. As detailed in anexample hereinbelow, I have successfully applied the molten agentdirectly to the jacket. Curing agents can also be applied as suspensionsor emulsions, such as water suspensions or emulsions.

- condensate drain 341 but a hot inert pressurized gas such as nitrogenor helium may be used instead of steam within the scope of my invention.The latter method has the advantage that it is easier to keep theapplication chamber relatively cool. To counteract the pressure in thetube 29 a pump 36 is used to meter the vulcanizing agent into he chamber28. In lieu of the continuous process shown, the entire cable can beremoved to a curing chamber after the surface has been coated withperoxide in solution. I have found that acetone works well as a solventfor di-a-cumyl peroxide in thisapplication' but other solvents, whichhave been known, can also be used. v

EXAMPLE 1 A cable made in accordance with the description of FIG. 1,hereinabove, with a jacket 13, 0.080 inch thick was brush coated with a50 weight percent solution of di-a-cumyl peroxide in acetone. Theacetone permitted to evaporate. Thereupon the cable was vulcanized in asteam chamber at 225 pounds per square inch in pressure for one minute.Specimens cut from the outer surface of the jacket with thicknesses from0022-0032 inch were found to have an average-tensile strength of 1,586psi compared to atensile strength of 1,373 psi for an unvulcanizedjacket specimen and an average elongation of 183 percent compared to 327percent for an unvulcanizedspecimen.

EXAMPLE-,2 i

A length of the cable was coated with molten dia-cumyl peroxide at 75psi. Specimens 0.020-0.024 inch thick were slicedfrom. the outer surfaceof the jacket and found to average 2,783 psi tensile strength and have120 2 percent elongation. I Specimens 0.0l80.020 inch thick were thensliced from the underlying thickness of jacket for testing to determinethe depth of diffusion of the curing agent. These averaged 2,870 psi intensile strength and 207 percent elongation. The much greater elongationindicates presence of some thermoplastic composition at the deeperlevel.

EXAMPLE 3 A cable with 250 MCM conductor, a 0.175 inch wall ofcross-linked polyethylene insulation and an 0.080 inch thick chlorinatedpolyethylene, ethylene ethyl acrylate jacket was coatedwith di-a-cumylperoxide and cured .in steam at 200 psi for 1 minute. A specimen0.0l50.020 inch thick was sliced from the outer surface of the jacketand found to have a tensile strength of 2,056 psi and an elongation of162 percent. The conductor of a length of this cable was raised to 135Cfor 24 hours to determine if the expansion of the insulation would causecracks to appear in the jacket. No cracks appeared. The conductortemperature was raised to 155C for 24 hours without causing any cracksto appear in the jacket. The conductor was raised to 200C for 1 hourwithout causing any cracks to appear over the drain wires. Acircumferential crack did appear next to a holding clamp. Forcomparison, cracks appeared over the drain wires of a cable with anunvulcanized jacket at a conductor temperature of 135C after 2 hours.

l have invented a new and useful cable and method of making the same ofwhich the foregoing description has been exemplary rather thandefinitive and for which I desire an award of Letters Patent as definedin the following claims.

lclaim:

1. An electric cable comprising:

A. a conductor,

B. a wall of dielectric polymeric composition surrounding saidconductor,

C. a jacket of polymeric composition surrounding said wall, comprising:

1'. a thermoplastic inner surface directly adjacent to said wall,'and'2. a vulcanized outer surface, said jacket being homogeneous save for adecreasing degree of vulcanization from said outer toward said innersurface. I

2. The cable of claim 1 wherein said wall comprises polyethylene.

3. The cable of claim 1 wherein said wall comprises vulcanizedpolyethylene.

4. The cable of claim 1 wherein said jacket is semiconducting.

Su'lhe cable of claim 1 whereinsaid jacket comprises chlorinatedpolyethylene.

6. The cable of claim 3 wherein said jacket comprises chlorinatedpolyethylene.

7. The cable of claim 4 comprising drain wires embedded within saidjacket.

8. The method of making'a jacketed cable comprising the steps of; v

A. extruding a polymeric jacket free from vulcanizing agent around aninsulated cable core,

B. applying a vulcanizing agent to the surface of said jacket, saidagent being sufficient to vulcanize said surface but insufficient tovulcanize the entirety of said jacket,

C. heating said cable thereby vulcanizing said surface and diffusingsaid agent into said jacket.

9. The method of claim 8 wherein said vulcanizing agent is applied tosaid jacket in solution.

10. The method of claim 8 wherein said jacket compriseschlorinatedpolyethylene blended with ethylene ethyl acrylate copolymer.

11. The method of claim 8 wherein said agent comprises an organicperoxide.

12. The method of claim 8 wherein said agent comprises di-a-cumylperoxide.

13. The method of claim 9 wherein said agent comprises di-a-cumylperoxide.

14. The method of claim 9 wherein said solution comprises acetone andsaid agent comprises di-acumyl peroxide.

15. The method of claim 8 wherein said heating is effected underpressure.

16. The method of claim 15 wherein said heating is effected bypressurized steam.

17 The method of claim 15 wherein said agent is applied to said jacketin solution.

18. The method of claim 15 wherein said jacket comprises chlorinatedpolyethylene blended with ethylene ethyl acrylate copolymer.

19. The method of claim 15 wherein said agent comprises in organicperoxide.

20. The method of claim 15 wherein said agent comprises di-a-cumylperoxide.

21. The method of claim 17 wherein said agent comprises di-a-cumylperoxide.

22. The method of claim 17 wherein said agent comprises di-a-cumylperoxide and said solution comprises acetone.

23. The method of claim 15 wherein said agent is ap plied within achamber containing said pressure.

24. The method of claim 16 wherein said agnt is applied within a chamberfolf said steam.

l 1 k B

2. a vulcanized outer surface, said jacket being homogeneous save for adecreasing degree of vulcanization from said outer toward said innersurface.
 2. The cable of claim 1 wherein said wall comprisespolyethylene.
 3. The cable of claim 1 wherein said wall comprisesvulcanized polyethylene.
 4. The cable of claim 1 wherein said jacket issemiconducting.
 5. The cable of claim 1 wherein said jacket compriseschlorinated polyethylene.
 6. The cable of claim 3 wherein said jacketcomprises chlorinated polyethylene.
 7. The cable of claim 4 comprisingdrain wires embedded within said jacket.
 8. The method of making ajacketed cable comprising the steps of: A. extruding a polymeric jacketfree from vulcanizing agent around an insulated cable core, B. applyinga vulcanizing agent to the surface of said jacket, said agent beingsufficient to vulcanize said surface but insufficient to vulcanize theentirety of said jacket, C. heating said cable thereby vulcanizing saidsurface and diffusing said agent into said jAcket.
 9. The method ofclaim 8 wherein said vulcanizing agent is applied to said jacket insolution.
 10. The method of claim 8 wherein said jacket compriseschlorinated polyethylene blended with ethylene ethyl acrylate copolymer.11. The method of claim 8 wherein said agent comprises an organicperoxide.
 12. The method of claim 8 wherein said agent comprises di-Alpha -cumyl peroxide.
 13. The method of claim 9 wherein said agentcomprises di- Alpha -cumyl peroxide.
 14. The method of claim 9 whereinsaid solution comprises acetone and said agent comprises di- Alpha-cumyl peroxide.
 15. The method of claim 8 wherein said heating iseffected under pressure.
 16. The method of claim 15 wherein said heatingis effected by pressurized steam.
 17. The method of claim 15 whereinsaid agent is applied to said jacket in solution.
 18. The method ofclaim 15 wherein said jacket comprises chlorinated polyethylene blendedwith ethylene ethyl acrylate copolymer.
 19. The method of claim 15wherein said agent comprises in organic peroxide.
 20. The method ofclaim 15 wherein said agent comprises di-Alpha -cumyl peroxide.
 21. Themethod of claim 17 wherein said agent comprises di-Alpha -cumylperoxide.
 22. The method of claim 17 wherein said agent comprisesdi-Alpha -cumyl peroxide and said solution comprises acetone.
 23. Themethod of claim 15 wherein said agent is applied within a chambercontaining said pressure.
 24. The method of claim 16 wherein said agentis applied within a chamber for said steam.